Improving the accuracy of circuit has been a requirement of analog circuits for a number of years. Zener zap trimming (both in the reverse and forward mode) has been used for years in improving the accuracy of circuits by adding or removing parallel impedances. Zener diodes operating in an untrimmed state act substantially as an open circuit. However, when sufficient amounts of current are supplied to a zener diode, the diode can be trimmed so as to close the open circuit and provide a reduced impedance across the zener diode. Adding these trimmed zener diodes in parallel allows the fine tunning of impedances on an electronic circuit. Other methods used for improving accuracy include metal link and poly link blowing. However, as technology and the semiconductor industry shifts towards submicron processes, the techniques for trimming zener diodes are not available or do not provide effective results.
However, a device called a degenerative zener diode has been developed which may be available in submicron processing. This PN diode structure acts as a high value resistor when it is in an untrimmed state and a low value resistor once a large (i.e., 100 mA) current has been passed through the zener. Circuitry surrounds this structure which senses whether the high or low resistance mode of the zener exists and uses this knowledge to selectively adjust the performance of analog circuits.
In prior art techniques for trimming degenerate zener diodes, a fixed current from a current source flows through the degenerate zener diode. If the voltage drop exceeds a given threshold, then the degenerate zener diode is considered untrimmed and no discrete changes are made to the circuitry requiring precision enhancement. Conversely, if the voltage is less than the threshold, then the degenerate zener is trimmed and a discrete change to the accuracy is permanently incorporated.
The problem with the prior art techniques is that the impedance of the degenerative zener diode in the untrimmed mode is not well defined and varies. Further, the impedance of untrimmed degenerative zener diodes are sensitive to semiconductor process variations resulting in variations in the impedance. This variation results in false determinations of the state of the degenerative zener diode and thus results in incorrect adjustments in the accuracy of circuits employing degenerative zener diodes.